Novel solid phase synthesis of a visible-light-active p-n heterojunction CuS/ZnS photocatalyst for wastewater treatment applications

Abstract

Water contamination by toxic organic chemicals is a major global environmental concern leading to photocatalytic technologies applied in wastewater treatment. The aim of this work was to investigate a new, simple one-pot combustion synthesis technique for creating sulphur- based CuS/ZnS p-n heterojunction nanocomposite photocatalysts. The study examined the photocatalytic activity and reusability of these nanocomposites in removing rhodamine B (RhB) dye from aqueous systems under visible light irradiation. Rhodamine B is an azo dye that is widely applied in processing operations such as the colouring process in textile industries which provides significant socioeconomic benefits; however, its minute traces in water antagonistically affect the environment and all life forms.

In this study, a novel heterointerface strategy is proposed for synthesising p-n heterojunction nanoporous agglomerate nanocomposites. This approach involves a simple one-pot one-step combustion method to that attunes the morphology and band gap energy of visible-light-induced nanomaterials. Various characterisation techniques were employed to analyse the physicochemical properties of the synthesised nanocomposite materials. X-Ray Diffraction (XRD) was used to ascertain the crystallinity and purity of the synthesised materials, while X-Ray Fluorescence (XRF) was utilised to determine the composition of the photocatalyst. To confirm the morphology and elemental chemical composition of the synthesised materials, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy-Dispersive X-Ray Spectroscopy (EDS) were conducted. Braunauer-Emmett-Teller (BET) analysis was employed to measure the surface area and pore size distribution of the materials. Furthermore, the optical properties, including the photo absorption range and band gap energy of the synthesised nanocomposite materials, were determined using Ultraviolet-Visible spectroscopy (UV-vis).

Several intrinsic reaction parameters affecting the photodegradation process were systematically varied to determine the optimal conditions, including the catalyst composition (CuS, ZnS, and CuS/ZnS), catalyst loading (ranging from 0 to 15 gL -1 ), initial solution pH (ranging from 1 to 13), and initial pollutant concentration (varying from 5 to 100 ppm). The experimental findings revealed that a binary CuS/ZnS catalyst, loaded at 10 gL -1 and with a pH of 5, achieved an impressive 97 % degradation of a 5 ppm RhB dye following 270 minutes of visible light exposure. These results highlighted the significant enhancement in photocatalytic degradation efficiency when pristine ZnS is coupled with highly photosensitive CuS. Specifically, the degradation efficiency improved from 67 % to 97 % within 4 hours of solar irradiation.

Moreover, it is noteworthy that the Langmuir-Hinshelwood kinetic model demonstrated the best fit to the data when a loading of 10 gL -1 was employed, yielding an impressive R 2 value of 0.99 and a maximum rate constant (k max ) value of 0.0186 min -1 indicative of pseudo-first-order kinetics rates. Additionally, this composite catalyst exhibited remarkable chemical stability and reusability, as it achieved 83 % RhB dye removal after five recycling runs. Further investigations involving scavenger tests identified the photogenerated holes (h + ) and superoxide free radicals (•O 2 ) as the primary reactive species responsible for degradation process. This comprehensive study provides valuable insight into the design of highly efficient nanomaterials for removing organic pollutants from wastewater, and a possible reaction mechanism is proposed.